1. Field of the Invention
The present invention relates to a Computed Tomography (CT) Explosives Detection System (EDS) for inspecting baggage for explosives or other contraband. More particularly, it relates to novel Computed Tomography (CT) scanners and processes to resolve threat alarms.
2. Discussion of Related Art
Following the terrorist attacks on Sep. 11, 2001, the United States government decided to implement additional airport security. One of the security measures which were to be implemented was inspection of all checked baggage for explosives. Implementation of this security measure using existing technology will be cumbersome and expensive.
Known Explosives Detection Systems (EDS) utilize either x-ray or computed Tomography (CT) technology to create an image of the contents of a bag.
The Transportation Security Act of 2001 significantly changed the manner in which airports must screen checked baggage. As of December 2001, all checked baggage must be screened for explosives by a technology certified by the newly formed Transportation Security Agency (TSA). To date, the only technology certified by the TSA is Computer Tomography (CT).
A Computed Tomography (CT) machine has been designed to perform automated explosives detection for passenger baggage, parcels, mail, and small cargo prior to loading onto an aircraft. CT technology has been proven to successfully meet the US FAA Certification requirements for automated explosives detection (EDS) in airline checked baggage. Most modern CT machines incorporate a rotating ring or “gantry” on which the X-ray source and detectors are mounted. The gantry rotates continuously, moving the X-ray source and detectors around the object under inspection to make cross sectional “slice” images through the object.
Given the current design of CT technology, the baggage scanners are quite large and cannot process baggage quickly. FIG. 1 is a cross sectional view of a conventional CT scanner 10. The CT scanner 10 includes a gantry 11 surrounding a tunnel 20. A conveyor (not shown) moves baggage through the tunnel 20 for scanning. The gantry 11 rotates about the tunnel, producing one slice of data for each rotation. An x-ray source 30 produces a narrow angle beam 40. A detector 31 is positioned on the gantry 111 to intersect the x-ray beam 40 passing through the tunnel. The detector 31 may consist of multiple detectors which are located equal distances from the x-ray source. The x-ray source 30 and detector 31 must be sized and positioned so that the entire tunnel falls within the x-ray beam. The data from the detector is analyzed using a computer to generate a three-dimensional representation of the contents of the baggage being scanned. The conveyor must move at a slow speed to generate sufficient data for reconstruction of the contents of the baggage from the slices.
Furthermore, the use of CT technology as currently implemented has significant limitations. Specifically, a significant number of false positives (false alarms) are automatically generated by this technology. Anywhere from 10–40% of the bags that are screened alarm current machines. These bags need to be further reviewed to determine whether an actual threat is present. The TSA has suggested new procedures for operators to review the images produced by a CT machine and clear some bags that do not present a threat. Even with these new procedures early tests have shown that anywhere from 8–15% of the bags screened can not be resolved by the machine and the operator. This creates an enormous operational problem for airports.
Under TSA protocol any bag that can not be cleared either automatically by the certified machine or by an operator under their new procedures, must be opened and inspected. The bag opening time varies from about 1.5 minutes up to 20 minutes depending on the location of the bag, the location of the passenger, number of threats in the bag, local airport procedures for opening and inspecting the bags, and whether the bag is locked or not. Not only is the bag opening process time and labor consuming but is intrusive to the privacy of the passenger. It is desirable for airports to achieve as low as possible bag opening rate while maintaining high detection standards. The goal of most airports is to achieve a bag opening rate of under 1% of the screening bags.